The present invention relates generally to a method of vacuum deposition and curing of oligomers and resins useful for making multilayer laminate structures from polymers or polymers in combination with other materials. More specifically, the present invention relates to forming solid polymer laminate layers under vacuum. Additional layers of polymer or non-polymer material may be added under vacuum as well.
As used herein, the term xe2x80x9cmonomerxe2x80x9d is defined as a molecule of simple structure and low molecular weight that is capable of combining with a number of like or unlike molecules to form a polymer. Examples include but are not limited to simple acrylated molecules, for example hexanedioldiacrylate, tetraethyleneglycoldiacrylate; styrene, methyl styrene; and combinations thereof. Monomer molecular weight is generally 1000 or less except for fluorinated monomers of about 2000 or more. Monomers may be combined to form oligomers but do not combine to form other monomers.
As used herein, the term xe2x80x9coligomerxe2x80x9d is defined as a compound molecule of at least two monomers that is radiation curable. Oligomer includes low molecular weight resins. Low molecular weight is herein defined as about 1,000 to about 20,000 exclusive of fluroinated monomers. Oligomers are usually liquid or easily liquifiable. Oligomers do not combine to form monomers.
As used herein the term xe2x80x9cnon-polymerxe2x80x9d includes but is not limited to inorganic materials for example metal, oxide, nitride, carbide, fluoride and combinations thereof.
As used herein, the term xe2x80x9cresinxe2x80x9d is defined as a compound having a higher molecular weight (generally greater than 20,000) and generally solid with no definite melting point, for example polystyrene, epoxy polyamine, phenolic, acrylic resin for example polymethylmethacrylate and combinations thereof.
As used herein, the term xe2x80x9cmaterialxe2x80x9d is inclusive of oligomer, resin, oligomer plus monomer, resin plus monomer and combinations thereof, but exclusive of monomer.
Laminate structures are used in many applications including but not limited to electronic devices, packaging material, and solar reflectors. Laminate structures in electronic devices are found in devices including but not limited to circuit elements and electrochromic devices wherein conductive polymer layers are combined and may include a non-polymer layer. Electrochromic devices include but are not limited to switchable mirrors and switchable windows. Circuit elements include active elements, for example fuel cells and batteries, and passive elements, for example capacitors.
Presently, many laminate structures are made with solid polymer laminate layers. In packaging material and solar reflectors, a metal layer may be added to enhance optical reflectance. In electronic devices, a metal layer may be added to enhance electrical conductivity. In packaging material and solar reflectors, it is not necessary that the polymer layer or layers be conductive, whereas in electronic devices, especially batteries, the polymer layers must be conductive to act as electrolytes, anodes, and cathodes. Certain polymers when doped with selected salts are known to make suitable solid polymer ion conductive layers. Polymers known to be useful include but are not limited to polyethyleneoxide, polypropyleneoxide, polyorgansulfides, and polyanaline. Suitable salts include but are not limited to lithium salts, for example lithium perchlorate, and lithium hexafluoroarsenate. Although the anode, cathode, and electrolyte layers may all be of solid polymer material, when making a lithium polymer battery, it is preferred to have a layer of lithium metal as an anode.
Other polymers having added compounds, including but not limited to conductive powders and dyes, may be made by the present invention.
Presently, mass production of polymer and non-polymer laminate structures used for electronic devices, and especially batteries, relies upon assembling preformed layers of polymer with a thin metal foil. Polymer layers are formed in production quantities by depositing in an atmosphere a thin layer of a liquid mixture containing monomer, oligomer, resin and usually combinations thereof onto a moving substrate that carries the liquid material layer while and until it is cured. Many means for forming polymer layers in an atmosphere are available, including but not limited to physical or mechanical liquid-material spreading apparati; for example, roll coaters, gravure roll coaters, wire wound rods, doctor blades, and slotted dies.
Vacuum deposition of monomer is shown in U.S. Pat. No. 5,260,095 using the physical or mechanical liquid-monomer spreading apparati previously set forth.
In addition means for evaporation and deposition of a monomer vapor, for example polymer multilayer deposition has been done in a vacuum as described in U.S. Pat. No. 5,681,615.
In any means having a moving substrate, the substrate has a velocity different from a nozzle or bath that deposits the liquid material onto the substrate. Hence, the term xe2x80x9cmoving substratexe2x80x9d as used herein excludes a situation wherein there is no relative motion or velocity differential between substrate and liquid material dispensing means.
The polymer multilayer deposition technique is distinct from liquid-monomer spreading techniques because polymer multilayer deposition requires flash evaporation of the monomer. First, a monomer is atomized into a heated chamber that is under vacuum. Within the heated chamber the monomer droplets are evaporated, then exit the heated chamber, and monomer vapor condenses upon a substrate and is subsequently cured.
Curing may be done by any means including but not limited to heat, infrared light, ultraviolet light, electron beam, and other radiation.
When fabricating a battery, several techniques are used to combine a thin metal layer with a conductive polymer layer. One technique of battery fabrication is to combine a metal foil with a conductive polymer layer by press bonding a metal foil layer to a solid conductive polymer layer. Another technique is to spread uncured liquid material onto a metal foil and subsequently cure the liquid material to form a solid conductive polymer layer. Use of metal foil, especially lithium metal foil, results in minimum metal thicknesses of from about 1.5 mils (40 micrometers) to about 2 mils (50 micrometers).
Other battery fabrication techniques include making a thin metal layer by sputtering, plating, or vacuum depositing metal onto a metal substrate. A conductive polymer is then placed in contact with the metal. Either solid conductive polymer or uncured liquid material may be brought into contact with the metal to form the battery. Polymer laminate structures, including but not limited to batteries, are made by a procedure wherein individual layers are sequentially and separately formed then combined.
The performance and lifetime of polymer/polymer and polymer/non-polymer laminate structures depend upon the quality of bonding between laminate layers. Bonding quality is affected by the presence of small, even microscale, areas of non-bonding at an interface between laminate layers. The bonding is especially critical between dissimilar layers; for example, polymer and metal layers. In batteries, reduced bond quality between polymer and lithium metal layers results in greater internal resistance of a battery produced with the laminate material and potential for xe2x80x9chot spotsxe2x80x9d upon recharging. In any structure, another problem with bonding dissimilar materials is chemical interaction between the materials. Areas of non-bonding can enhance chemical interaction because they may contain non-inert species or provide different surface characteristics at a boundary between bonded and unbonded areas.
Bonding between layers is therefore of great importance and is enhanced by several means, including but not limited to mechanical presses, and application of a second layer as a liquid with subsequent solidification upon a first solid layer at atmospheric pressure. The difficulty with these methods is that the low cost assembly of pressing or liquid application leads to low quality bonding as identified in U.S. Pat. No. 4,098,965, issued Jul. 4, 1976, to Kinsman, column 1, lines 47-50, wherein he states xe2x80x9c[g]ases usually air, [that] are included in the void regions of the battery during assembly . . . xe2x80x9d.
It is of great interest to those skilled in the art, then, to make batteries and other products of polymer layers having high bond quality, low permeability, while making them in a cost effective manner.
The present invention is the formation of solid polymer layers under vacuum. More specifically, according to a first aspect of the present invention, the use of xe2x80x9cstandardxe2x80x9d polymer-layer making equipment, is generally used in an atmospheric environment in a vacuum, and degassing the material prior to injection into the vacuum. Additionally, other layers of polymer or non-polymer material may be vacuum deposited onto solid polymer layers. Non-polymer materials include but are not limited to inorganic materials for example metal, oxide, nitride, carbide, fluoride and combinations thereof.
Advantages of forming polymer layers in a vacuum include use of less to no photoinitiator for curing, faster curing, and fewer impurities in the polymer. Further advantages are improvement in material properties, including no trapped gas, resulting in greater density and reduced material wetting angle that facilitates spreading of the material and provides a smoother finished surface.
According to a second aspect of the invention, fabrication of laminate structures are carried out nearly simultaneously within a single vacuum chamber.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following detailed description.